A. Technical Field
This invention relates generally to integrated optical components, and more particularly, to integrated optical combiners and decombiner, such as a photonic integrated circuit (hereinafter, “PIC”), having a plurality of different input/output configurations.
B. Background of the Invention
The importance of high speed networks, such as optical WDM networks, is well understood by one skilled in the art.
FIG. 1 illustrates an exemplary WDM network in which a plurality of wavelength channels is communicated between network nodes. A transmitting node comprises an optical combiner 5 having a plurality of inputs 4 on which wavelength signals are provided. The optical combiner 5 optically multiplexes these wavelengths into a single WDM signal and outputs this WDM signal into a piece of optical fiber 6. An optical fiber span, coupling the networking nodes, may include multiple amplifier nodes 7 that may re-amplify, re-shape, re-time or otherwise process the WDM signal.
A receiving node comprises an optical decombiner 8 having an input on which the WDM signal is received. The optical decombiner optically demultiplexes the WDM signal into its component wavelength signals and outputs these wavelength signals on a plurality of outputs 9. These outputs may be coupled to a plurality of photodetectors that convert the wavelength signals into corresponding electrical signals.
These optical combining and decombining components are traditionally located in relatively large and expensive modules within optical transponders. There is a trend in the optical component field to integrate active and passive optical components on a single monolithic chip, such a silica/SiO2-based chips or InP-based chips in order to reduce the cost of these components. For example, certain optical components may be integrated within a single transmitting photonic integrated circuit (hereinafter, “TxPIC”) chip having multiple signal channels, each channel having a modulated optical source for producing channel signals of different wavelengths. A description of an exemplary TxPIC may be found in the published article of Nagarajan et al. entitled, “Large-Scale Photonic Integrated Circuits”, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 11 (1), pp. 50-65, January/February 2005. (See also U.S. patent application, entitled “Transmitter Photonic Integrated Circuits (TxPIC) and Optical Transport Networks Employing TxPICs,” U.S. Patent Application Publication No. 20030095737, which is incorporated herein by reference in its entirety)
A receiving photonic integrated circuit (hereinafter, “RxPIC”) chip is also known in which optical receiving components are integrated into a single monolithic chip. (See U.S. patent application, entitled “Optical Signal Receiver Photonic Integrated Circuit (RxPIC), an Associated Optical Signal Transmitter Photonic Integrated Circuit (TxPIC) and an Optical Network Transmission System Utilizing these Circuits,” U.S. Patent Application Publication No. 20040033004, which application is incorporated herein by reference in its entirety) In the deployment of an optical receiver, such as an RxPIC, it is oftentimes necessary to provide pre-amplification functionality for the WDM signal prior to it entering demultiplexing and detecting elements. This pre-amplification may be necessary to compensate for attenuation and other losses of a WDM signal that has propagated along an optical link. One function of the pre-amplification is to ensure that the WDM signal power is within a preferred range so that accurate demultiplexing and detection may be performed.
The WDM signal may also experience polarization dependent effects (hereinafter, “PDEs”), such as polarization dependent loss (hereinafter, “PDL”) and polarization dependent gain (hereinafter, “PDG”), where the polarization modes of the signal propagating along the fiber randomly vary in relation to time and wavelength. When the WDM signal reaches the receiver, its polarization components may have unknown magnitudes and phases relative to one another.
Optical amplifiers, such as those optical amplifier nodes previously described, are positioned along the link to compensate for signal attenuation and other degradations. These optical amplifiers have traditionally been a fiber amplifier, such as an EDFA; however, semiconductor optical amplifiers (hereinafter, “SOAs”) may also be available and designed to provide sufficient gain and low PDEs at designed operational gain level. SOAs are generally considered preferable over EDFAs because EDFAs are bulky, expensive and require optical laser pumps to provide optical stimulation to their rare earth content therein. On the other hand, SOAs are very small in comparison, are amendable to integration, and are pumped by a bias current within the SOA substrate.
With the advent now of integrated optical chips, such as RxPIC and TxPIC chips, it would be desirable to extend integration of the optical amplifiers into the receiver and transmitter chips. However, the integration and sensible realization of these optical amplifiers present certain difficulties. For instance, WDM signals experience different types of degradations in their propagation in metro- and long-haul transmission fibers. These transmission fibers are generally different lengths, which may vary between 10 km-100 km from one node to the next. The pre-amplification requirements of a WDM signal may vary depending on the length of an optical link on which the WDM signal was transmitted and the particular profile of that link. The amount of gain may also depend on how much noise is on the WDM signal and the particular detectors used within the receiver. As a result, it would be preferred to manufacture an integrated receiver circuit that has an integrated input amplifier functionality which can provide different levels of gain and different gain characteristics to the WDM signal.
While a SOA can be biased to provide different levels of gain, the variance of its gain level may cause changes in its PDG so that it will no longer be effective in providing an appropriate gain to certain incoming WDM signals. In fact, if a gain is improperly applied, it may result in further deterioration of the WDM signal. Accordingly, it is important that gain functionality be integrated in which the gain characteristics, beyond just an average applied gain, may be adjusted relative to the optical system and environment in which the chip will function.
There are also situations where it may be desirable to have an integrated receiver chip comprise an input that does not have integrated pre-amplification elements, such as an integrated SOA. Pre-amplification requirements may be obviated by using a link profile to transmit a WDM signal at a particular power profile so that it is received within a desirable power range. Furthermore, external pre-amplification elements may be provided that apply a gain to the WDM signal, obviating the need for integrated amplification functionality.
These same issues may be applied to integrated transmitting chips, such as a TxPIC, in which post amplification may be required. In particular, an amplifier may be required to apply a gain to a WDM signal that was generated by an optical combiner. This post-amplification may be done by an integrated SOA or an external SOA depending on the design of the optical system.
Accordingly, what is needed is a configurable optical combiner and decombiner that may be configured to operate in numerous different types of optical systems and provide both integrated amplified and unamplified waveguide paths.